在If…End If选择结构中，允许可嵌套的深度是( )。

A．最多32层

B．最多64层

C．最多256层

D．没有严格限制

— Why didn’t you try your best to get on the subway?

— I tried to, but it started moving ______ I could get on it.

A．before

B．after

C．since

D．if